Rbe over temperature?

[Terry Michaels]

Message text written by INTERNET:grichter at asapnet.net
>I am wondering if transistor Rbe is temperature dependent?

There seems to be less Rbe compensation required at lower temperatures. The
oscillators I calibrated during the summer seem to sharp at C7 now,
although
the C2-C3 range still looks OK. Calibrated at 80 degF tested at 65 degF. I
am only seeing a small apparent slope error.

The transistor gain goes down with increasing temperature, so the tempco
increases the gain to the drive stage as temperature goes up. As I recall
that silicon has a theoretical -3300 ppm/C slope change. The tempco has a
+3500 ppm/C indicating a possible overcompensation of +200 ppm/C for
INCREASING temperature. So the high end should go sharp as temperature
increase.

The fact that they go sharp with DECREASING temperature I can only explain
if the Rbe is decreasing with temperature?

Any thoughts?

<

Hi Grant:

I'm willing to bet the bulk resistance of silicon has a positive
temperature characteristic, like most other conductors and semiconductors. 
So, your observation makes perfect sense.  OTOH, I don't think it has
anything to do with the gain of the transistor.  Also, it can't be because
of the difference between 3300 and 3500 ppm, that is a scale factor issue,
so if that was the cause you would see an incorrect scale factor setting at
all pitch ranges, not just at C7.

Terry Michaels